The present invention is related to the positioning control of the read-write head of a data recording disk unit. More particularly, the head positioning control of the present invention is a sector servo system, and is directed to the control system of the track following mode.
In a disk unit, accurate positioning of the head at a target track is an essential factor in providing the disk unit with a large capacity, and quick positioning of the head at the target track is also an essential factor in providing the disk unit with a high speed. In a high-density disk unit, in order to accomplish the foregoing, a voice coil motor is used as a driving device, positional information is previously written on the disk surface, the positional information is detected by the head and fed back, and the above driving device is driven by a closed-loop servo system to perform the positioning of the head. The head positioning operation is largely classified into two modes. One is the velocity control mode, and the other is the track following mode. In the velocity control mode, the velocity of the head is controlled to move the head to the neighborhood of a target track within the minimum time. After the head reaches the above-mentioned neighborhood of the track, control goes to the track following mode. In the track following mode, control is made so as to zero out the position error between the head and the target track, whereby the head is positioned at the target track with high precision, completing the head positioning operation. The description of these head positioning operations is given in R.K. Oswald, "Design of a Disk File Head-Positioning Servo", IBM Journal of Research and Development, November 1974, pp. 506-512.
As disk units are made small and provided with a large capacity, the sector servo system (or buried servo system) is attracting notice as a head positioning system suitable for small-sized and large-capacity disk units. This system has the following advantages as compared with the conventional servo surface servo system which uses a dedicated servo surface.
Since the servo positional information is placed in the sectors on the data surface instead of being recorded by occupying one surface of a disk medium such as the servo surface, the utility factor of the data area of the total disk surface in the disk file is high.
Since the servo positional information is placed on each surface of the disk, it may be possible to neglect the deviation of the head from the target track position (off-track) which is caused by the inclination of the spindle for rotating the disk medium due to heat or the like.
The layout of the servo positional information area and the data area on the surface of the disk medium in this system is shown in FIG. 5. As is apparent from FIG. 5, the positioning control of this sector servo system adopts sampled data control, as compared with the conventional servo surface servo system which is a continuous system control, because the servo positional information is in the sectors and its detection becomes discontinuous. One method of head positioning control of this sector servo system is introduced in "HIGH TRACK DENSITY FOR MAGNETIC DISK DRIVE WITH AN `EMBEDDED SERVO` POSITIONING SYSTEM" by C. Maury, IEEE TRANSACTION ON MAGNETICS, VOL. MAG-17, NO. 4, JULY 1981, pp. 1396-1402.
As the speed and cost reduction of microprocessors have proceeded, it has become possible to make the processors perform various calculations to carry out estimative control. To perform estimative control, it is necessary to cause an arithmetic device called an estimator to process much of the servo data. One such method is disclosed in Japanese patent application No. 62-70561.
The track following mode in head positioning control is most important in allowing the head positioning to be performed with high precision and at a high speed. In performing the sampled servo data control, if it is assumed that the voice coil is driven with current, the driving current which is to flow after the present sampling and till the next sampling is denoted I.sub.n, the head position (distance to the target track) detected at the present sampling is X.sub.n, and the head velocity is V.sub.n. The following relation is applied. EQU I.sub.n =-AX.sub.n -BV.sub.n ( 1)
It is known to drive the voice coil with a driving current given by the equation (1) only. The values of constants A and B are optimized on the basis of the maximum current value, sample period and mechanical rigidity characteristics, depending on each application. When it is desired to position the head at the target track within a short time in the track following mode and terminate the head positioning operation, if it is assumed, for instance, that the sample period is T, the constant for converting acceleration to current is M, and A=M/T.sup.2 and B=3M/(2T), then the head is allowed to rest at the target track at the 3rd sampling. The movement of the head in this case is shown in FIG. 6. FIG. 6 represents normalization provided by assuming that the sampling interval is 1, the head position when entering the track following mode is 1, and the velocity of the head when entering the track following mode is -1.
The sampling and the head movement will be described referring to FIG. 4. The servo positional information is radially arranged from the center of the disk medium surface as shown. Since the disk medium is rotating at a fixed number of revolutions and the head is moved by the driving device toward the center of the disk medium, the locus of the head movement is as shown in the figure. In addition, since the disk medium is rotating at a fixed number of revolutions as mentioned above, the sampling interval becomes T which is constant. It is apparent that, if the number of sectors of one track is N and the number of disk revolutions is R per second, T=1/(N.times.R).
The area placed at the beginning of each servo positional information and erased by a d.c. current for instance, is detected, and a sector mark is output. The sector mark triggers the sampling gate for sampling the position signal. The sampling gate is opened for a certain predetermined time, during which the servo positional information is taken in. At the same time, the velocity signal is also sampled, providing X.sub.n and V.sub.n of the above equation (1). An arithmetic operation is performed using these to vary a driving current, thereby positioning the head at the target track.
In following a track, the driving device is subjected to a kind of force which is substantially fixed or is of a very low frequency, this force causes an off-track of the driving device, hence of the attached head from the target track. These disturbances are generated by the circulating air flow due to the disk rotation, the force due to the bending of the flexible cable connecting the head and the circuitry portion, gravity, etc. Factors causing the off-track exist in addition to the aforementioned disturbances, and one of them is the input voltage offset of the power amplifier supplying the driving device with a current, for instance. The off-track due to these largely affects the precision of head positioning.
Taking the aforementioned example, consideration is provided for the case that a disturbance of an acceleration .alpha. is applied to the driving device. That is, it is assumed that the actual acceleration of the driving device is added with .alpha.. Thus, .alpha.T.sup.2 n always remains at the position term, and an off-track error is small while .alpha. is small, but as .alpha. becomes large, the off-track error naturally becomes large, affecting the performance of the disk device. That situation is shown in FIG. 7. FIG. 7, similarly to FIG. 6, represents the normalization provided by assuming that the sampling interval is 1, the head position when entering the track following mode is 1, and the head velocity when entering the track following mode is -1.
In order to reduce such off-track due to disturbances, several measures have so far been taken.
One method is to use an estimator as shown in the above Japanese patent application No. 62-7051 or "OFFSET FORCE CORRECTION FOR DISK FILE" by J. P. Mantey, IBM Technical Disclosure Bulletin, Vol. 21, No. 5, October 1978, pp. 1792-1795. In this case, the amount of arithmetic operation of the estimator is large, which makes it difficult to adopt the estimator in low-cost, small-sized disk units. Further, since the method as shown in Published Unexamined patent application No. 59-146486 needs a current detector for detecting the driving current of a servo motor and also needs several analog circuits such as differential circuits to differentiate the detected velocity of the servo motor, it makes the hardware construction and operation complicated, and it is not suitable for small-sized disk units intended to simplify the circuit construction and operation and to attain cost reduction by digitization.